Preparation and Quality Control of [N-Methyl- 11 C]choline for Routine PET Application

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1 Radiochemistry, Vol. 45, No. 4, 2003, pp. 377!381. Translated from Radiokhimiya, Vol. 45, No. 4, 2003, pp. 342!345. Original Russian Text Copyright by Kuznetsova, Fedorova, Vasil ev, Simonova, Nader, Krasikova. ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ Preparation and Quality Control of [N-Methyl- 11 C]choline for Routine PET Application O. F. Kuznetsova*, O. S. Fedorova*, D. A. Vasil ev*, T. P. Simonova*, M. Nader**, and R. N. Krasikova* * Institute of Human Brain, Russian Academy of Sciences, St. Petersburg, Russia ** ARGOS Zyklotron GesmbH, PET-Zentrum LKH Klagenfurt NMSE, Klagenfurt, Austria Received September 10, 2002 Abstract-The goal of this study was to optimize the synthesis of [N-methyl- 11 C]choline, a radiopharmaceutical used in the diagnosis of brain tumors and prostate cancer with positron emission tomography (PET). The synthetic method is based on methylation with [ 11 C]CH 3 I of N,N-dimethylaminoethanol (DMAE) immobilized on the surface of a tc18 solid support (Waters). The optimal amounts of DMAE (25 ml in50 ml of ethanol) and tc18 (0.1 g) were found, providing a high radiochemical yield of the labeled choline (85%, corrected for radioactive decay) and radiochemical purity of more than 99.5%. After purification on the Sep-Pak Light cation-exchange cartridge (Accell Plus CM, Waters), the concentration of DMAE in the final product was 1.6 mg ml!1. For monitoring of DMAE in the final product, a simple and convenient HPLC method with an indirect UV detection providing sufficient sensitivity was proposed. Positron emission tomography (PET) is an advanced technique giving a number of opportunities for in vivo diagnosis of various types of tumors. The fluorinated glucose analog, [ 18 F]fluoro-2-deoxy- D-glucose (FDG), is the most commonly used radiopharmaceutical (RP) for evaluation of the metabolic activity of tumors. Because FDG is a radiotracer of glycolysis, its tissue uptake reflects the glycolytic activity, which correlates with the grade of the tumor. The lack of specificity to the tumor cells is the major limitation of this radiotracer. In addition, FDG has significant physiological uptake in the grey matter, which may be responsible for an uncertain tumor diagnosis [1]. Another metabolic tracer is L-[methyl- 11 C]methionine (L-MET), an analog of natural amino acid L-methionine. With L-MET, more contrast images of lesions are obtained as compared to FDG. However, a complex mathematical analysis of the data obtained is required to assign the tumor grade with L-MET [2]. A considerable recent interest has been focused on development of the principally new class of the RPs for PET oncology. In contrast to metabolic tracers (FDG, L-MET), these new RPs accumulate in tumor tissues owing to enhanced proliferation rate. In this connection, the derivatives of thymidine ([ 18 F]-3`-deoxy-3`-fluorothymidine [3]) involved in the DNA synthesis, and the analogs of choline ([N-methyl- 11 C]choline, [ 18 F-fluoromethyl]choline) were considered as the most promising candidates. The prospects for the use of labeled choline analogs in PET are based on the fact that choline is the precursor in the biosynthesis of phosphatidylcholine, the major component of cell membranes. Phosphatidylcholine levels are substantially increased in rapidly proliferating tissues. The radioactive analogs of choline are not accumulated by normal brain, providing high-contrast PET images of the tumors. In addition to brain tumors [537], labeled analogs of choline are considered as valuable RPs for PET diagnosis of prostate cancer [8, 9]. Whereas the synthesis of [ 18 F-fluoromethyl]choline is sophisticated [10], [N-methyl- 11 C]choline can be readily prepared by the well-known methylation technique using [ 11 C]methyl iodide [4, 11, 12] or [ 11 C]methyl triflate [13]. In the case of [N-methyl- 11 C]choline, the precursor, N,N-dimethylaminoethanol (DMAE), acts 3ND at the same time as the reaction medium and the base: BCH CH3 I + DBCH OH3CH 2 3CH 2 OH3CH 23CH 11 %%%%$ 23N3 CH 3. CH 3 CH 3 The major problems in preparing [N-methyl- 11 C]- choline are removal of excess DMAE from the reaction mixture and determination of the residual precursor in the formulated product, as DMAE may inhibit the incorporation of choline into the cell membranes [14]. The use of conventional HPLC technique with spectrophotometric detection is complicated by the lack of DMAE absorption bands in the UV range /03/ $25.00 C 2003 MAIK [Nauka/Interperiodica]

2 378 KUZNETSOVA et al. Fig. 1. Scheme of the synthesis and purification of [Nmethyl- 11 C]choline (method B). Eluents: (1, 3) aqueous, (2) ethanolic, and (4) [N-methyl- 11 C]choline in 0.9% NaCl. The refractive index detector is insufficiently sensitive for determination of low levels of DMAE in the final product ( mg ml 31 ). Therefore, the detection of DMAE was performed by the LC/MS technique [13] or with an electrical conductivity detector [15], as well as by GC [12]. The HPLC detectors mentioned above are expensive and not widely available for routine quality control of RPs, while GC technique does not always provide quantitative data for dilute aqueous solutions. Here we report a simple and convenient HPLC method for analysis of small amounts of DMAE using a standard UV detector and indirect spectrophotometry. The optimal conditions for the synthesis of [N-methyl- 11 C]choline with an Anatech RB-86 laboratory robot are described. EXPERIMENTAL Materials and chemicals. N,N-Dimethylaminoethanol (Aldrich) and choline chloride (Sigma) were used without additional purification. A Sep-Pak Light Accell Plus CM cation-exchange cartridge (W0259L3, Waters) was conditioned by passing 10 ml of water just before use and flushed with nitrogen. Disposable solid-phase extraction columns Sep-Pak Vac tc18, 6 CC (WAT036790, Waters) and Supelclean LC-18, 6 ml (57 054, Supelco) were used without conditioning. Preparation of radioactive precursors. Radioactive 11 C(T 1/2 = 20.4 min) in the form of [ 11 C]CO 2 was produced by the 14 N(p, a) 11 C nuclear reaction by irradiation of nitrogen gas with 17-MeV protons at a beam current of 40 ma (MC-17 cyclotron, Scanditronix, Sweden). [ 11 C]CH 3 I was prepared by the classical method based on reduction with lithium aluminium hydride, with subsequent distillation in a nitrogen flow; all the procedures were performed in the robotic station (Scanditronix) as described previously [16]. The gaseous product was purified by passing through a trap packed with ascarite and phosphorus pentoxide (50/50 by volume) placed in the distillation line. Radioactive synthesis was operated by a laboratory robot (Anatech RB-86, Sweden) installed in a hot cell (Van Gahlen, the Netherlands). The radioactivity was measured with a PTW Curiementor-2 isotope calibrator (Germany). Synthesis and purification of [N-methyl- 11 C]- choline. Method A. [ 11 C]CH 3 I delivered by a nitrogen flow of 40 ml min 31 was trapped in an ice-bath cooled 5-ml conical reaction vial containing 0.1 ml of N,N-dimethylaminoethanol (DMAE). Trapping was monitored by measuring the activity in the isotope calibrator, and the maximal value was attained within 537 min at 0oC. The reaction vial was sealed and kept in a heating block at 130oC for 5 min. After the reaction, DMAE was removed by distillation at the same temperature in a nitrogen flow for 233 min. The residue was dissolved in 5 ml of water, and the solution was passed through a Sep-Pak Light Accell Plus CM cartridge installed in the solid-phase extraction station of the robotic system. The cartridge was rinsed with 5310 ml of water, after which [N-methyl- 11 C]choline was eluted with 5 ml of normal saline (0.9% NaCl). Method B. [ 11 C]CH 3 I was passed in a nitrogen flow (40 ml min 31, 5 min) through a 1-ml disposable column packed with g of tc18 resin onto which ml of DMAE in ml ofc 2 H 5 OH had been applied (Fig. 1). The radioactivity of the column was measured, and a Sep-Pak Light Accell Plus CM cation-exchange cartridge was attached to the output. The joined column and cartridge were rinsed with 5 ml of water, 5 ml of ethanol, and 10 ml of water, after which [N-methyl- 11 C]choline was desorbed by elution with 5 ml of normal saline. Analytical conditions. HPLC analysis was performed using a Gilson 305 pump and a Rheodyne 7125 injector under the following conditions. System 1 was used for identification and evaluation of the radiochemical purity of [N-methyl- 11 C]choline. It included a Gilson 131 refractive index detector and a Beckman 170 radioactivity detector; Zorbax C8 column, mm, mobile phase M naphthalene-2-sulfonic acid sodium salt containing 0.05 M

3 PREPARATION AND QUALITY CONTROL OF [N-METHYL- 11 C]CHOLINE 379 Conditions of synthesis and purification of [N-methyl- 11 C]choline and content of DMAE in the final product (method B)* ÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ ³ ³ ³ Activity of fractions, mci (Fig. 1) ³ tc18, ³ DMAE/C 2 H 5 OH,³ ³ A[ 11 C]CH3 I ³ A product, ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ DMAE,** g ³ ml ³ on tc18, mci³ mci ³ 1: H 2 O, ³ 2: C 2 H 5 OH,³ 3: H 2 O, ³4: NaCl solu-³ mg ml!1 ³ ³ ³ ³ 5ml ³ 5 ml ³ 10 ml ³ tion, 5 ml ³ ÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄ 0.5 ³ 30/50 ³ 89.8 ³ 76.8 ³ 0.30 ³ 0.90 ³ 0.15 ³ 76.8 ³ ³ 25/75 ³ 39.2 ³ 34.3 ³ 2.60 ³ 2.30 ³ 0.25 ³ 34.3 ³ ³ 25/50 ³ 40.5 ³ 31.8 ³ 0.10 ³ 0.04 ³ 0.02 ³ 31.8 ³ 1.6 ÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ * All activities were corrected for the 11 C decay by the end of [ 11 C]CH 3 I uptake on tc18. ** DMAE content in the final product according to HPLC (system 2). H 3 PO 4, flow rate 1 ml min 31 ; injection volume 20 ml. The retention times of DMAE and choline chloride were and min, respectively; the retention time of sodium chloride was min. System 2 was used for analysis of the content of DMAE in the formulated product: indirect UV spectophotometery, Gilson 116 UV detector, 269 nm, inverted polarity, Zorbax SCX column, mm, mobile phase H 2 O/pyridine/CH 3 COOH (1000/0.05/0.1 by volume), flow rate 1 ml min 31, injection volume 10 ml. The retention time of DMAE was min. RESULTS AND DISCUSSION [ 11 C]CH 3 I methylation of N, O or S functions is the most common way to introduce 11 C into various organic molecules. In the synthesis of [N-methyl- 11 C]choline, the precursor, DMAE, acts as a solvent and base (see scheme). The methylation proceeds almost quantitatively at 130oC within 5 min. Purification by solid-phase extraction on a Sep-Pak Light Accell Plus CM cation-exchange cartridge, suggested in [6] and later used by the same group in their automatic module [11], allowed elimination of the traditional HPLC purification step [14] and simplified the synthesis. In this work, the method described in [11] was adapted to the Anatech RB 86 robotic system (method A). The decay-corrected radiochemical yield based on the activity of [ 11 C]CH 3 I trapped on the cartridge was in the range % within min synthesis time (preparation of the methylating agent is not included). The radiochemical purity was more than 99%. The losses of activity on Sep-Pak Light Accell Plus CM cartridge were no more than 0.5%. The analysis of the product by ion-pair HPLC with refractive index detection, according to the procedure described in [11] (system 1), revealed the presence of significant amounts of DMAE in the final product ( mg ml 31 ). Attempts to remove DMAE more exhaustively by increasing distillation time and nitrogen flow (the boiling point of DMAE is 133oC) were accompanied by large losses of the radioactivity on the walls of the reactive vial. As a result, the radiochemical yields were poorly reproducible. Therefore, we concluded that method A was unsuitable for routine application. At present, [on-line] reactions find growing use in radiochemical synthesis. With this technique, the precursor is immobilized on the surface of a solid material or on the inner surface of tubing, while the radioactive reagent is passed with a gas flow through the loaded cartridge or introduced as a solution. The advantages of [on-line] methods are minimal amounts of chemicals, fast reactions, and simplicity of automation, as it was recently demonstrated as applied to solid-supported synthesis of [N-methyl- 11 C]choline [12]. In that work, [ 11 C]CH 3 I in a flow of an inert gas was passed through the tc18 cartridge loaded with ml of DMAE. The reaction proceeded at room temperature, and the radiochemical yield was optimized by adjusting the flow of the methylating agent. [N-Methyl- 11 C]choline was purified by the previously described method [11] on a Sep-Pak Light Accell Plus CM cation-exchange cartridge connected after the tc18 cartridge. With a minimal amount of DMAE (40 ml) as a precursor, its residual content in the formulated product, as detected by GC analysis, was, on the average, 28 mg ml 31 [12]. In this study, the technique suggested in [12] was slightly modified (method B): DMAE was diluted with ethanol in various proportions (see table) to keep the amount of the precursor loaded as low as possible without decreasing the total volume of the solvent required for wetting of the resin. The Sep-Pak tc18 cartridge used in [12] was replaced by disposable Sep-Pak Vac tc18 CC 6-ml tube (Waters) packed with 0.5 g of tc18 and attached to the robotic station with a special adapter. Under these conditions (table, first two rows),

4 380 KUZNETSOVA et al. the concentration of DMAE in the final product was in the range mg ml 31, which is similar to the values reported in [12]. To decrease the DMAE content in the final product, the amount of tc18 was reduced to 0.1 g (table, last row). These conditions appeared to be optimal, since they provided the DMAE concentration as low as 1.6 mg ml 31 without decreasing the radiochemical yield. It should be noted that replacement of C18 packing material traditionally used in the synthesis and purification of RPs by tc18 seems to be important for radiochemical yield of the product. In our comparative studies using two disposable columns, Supelclean LC-18 6-ml disposable column (Supelco) and Sep-Pak Vac tc18 6 CC, packed with an equal amount of the material (0.5 g), the activity loss on the C18 resin was large, whereas on tc18 it did not exceed 0.5%. The radiochemical yield of [N-methyl- 11 C]choline corrected for the radioactive decay of 11 C (method B) was, on the average, 85%, with the radiochemical purity of the product exceeding 99.5%. The synthesis time starting from trapping of [ 11 C]CH 3 I was min. For routine quality control of PET RPs, simple and reliable analytical methods are highly demanded. HPLC with refractive index detection suggested in [6, 11] and discussed in [12] is an appropriate and fast method for controlling the radiochemical purity and identity of the labeled choline. However, the sensitivity of this method to DMAE is very low [12]. With the refractive index detector available in this study, the detection limit for DMAE (system 1) was about 100 mg ml 31, which is much higher than 0.2 mg ml 31 reported in [11]. As an alternative to HPLC with refractive index detection, the use of GC technique has been recently considered [12]. However, we failed to obtain the expected result with this method, since a small peak of DMAE was [taken away] into the negative area by a huge peak of water. Under these conditions, the precise integration of the DMAE peak area was difficult. To analyze low levels of DMAE, we used an HPLC method with indirect (contrast) UV detection. This method was used, for example, for analysis of opiates [17]; however, to our knowledge, it has not been reported for the quality control of [N-methyl- 11 C]choline. Under proper analytical conditions (system 2, Fig. 2) the DMAE peak was clearly distinguished from the large neighboring peak of sodium chloride, the major component of normal saline used for intravenous injection. The proposed method has a high sensitivity (1 mg ml 31 ); it is simple and does not require expensive detectors or chemicals. It is easily adaptable to various HPLC systems with Fig. 2. HPLC chromatogram for the analysis of DMAE in a solution of [N-methyl- 11 C]choline in 0.9% NaCl: Zorbax SCX column, mm, mobile phase H 2 O/pyridine/CH 3 COOH (1000/0.05/0.1 by volume), UV 269 nm, inverted polarity. UV detector available in a number of PET centers. It should be noted that the permissible concentration of DMAE in intravenous injections in humans is not prescribed in the European Pharmacopoeia, since this compound is not included in the group of toxic solvents (LD 50 for rats is more than 234 mg kg 31 ). The rational base for this limitation is that DMAE may inhibit the incorporation of labeled choline into the cell membranes [14]. From two methods evaluated in this study, method B proved to be preferable for routine PET application, since it allowed preparation of the final product containing much lower amounts of DMAE (1.6 mg ml 31, compared to 1.1 mg ml 31 ). To conclude, the robotic synthesis of [N-methyl- 11 C]choline based on methylation on the solid-phase tc18 support (method B) allowed production of this radiopharmaceutical in a high radiochemical yield and with high levels of radiochemical and chemical purity. The analytical HPLC method with indirect UV detection suggested in this study for the control of residual DMAE in the final product is highly sensitive, simple, and convenient, which makes it suitable for routine PET application. REFERENCES 1. Sasaki, M., Kuwabara, Y., Yoshida, T., et al., Eur. J. Nucl. Med., 1997, vol. 25, pp Skvortsova, T.Yu., Brodskaya, Z.L., Rudas, M.S., et al., Med. Vizual., 2001, no. 1, pp Schields, A.F., Grierson, J.R., Dohmen, B.M., et al., Nature Med., 1998, no. 11, pp Friedland, R.P., Mathis, C.A., Budinger, T.F., et al., J. Nucl. Med., 1983, vol. 24, pp Shinoura, N., Nishijima, M., Hara, T., et al., Radiology, 1997, vol. 202, pp

5 PREPARATION AND QUALITY CONTROL OF [N-METHYL- 11 C]CHOLINE Hara, T., Kosaka, N., Shinoura, N., and Kondo, T., J. Nucl. Med., 1997, vol. 38, pp Ohtani, T., Kurihara, H., Ishiuchi, S., et al., Eur. J. Nucl. Med., 2001, vol. 28, pp Hara, T., Kosaka, N., and Kishi, H., J. Nucl. Med., 1998, vol. 39, pp Kotzerke, J., Prang, J., Neumaier, B., et al., Eur. J. Nucl. Med., 2000, vol. 27, pp DeGrado, T.R., Coleman, R.E., Wang, S., et al., Cancer Res., 2001, vol. 61, pp Hara, T. and Yuasa, M., Appl. Radiat. Isot., 1999, vol. 50, pp Pascali, C., Bogni, A., Iwata, R., et al., J. Label. Comp. Radiopharm., 2000, vol. 43, pp Roivainen, A., Forsback, S., Gronroos, T., et al., Eur. J. Nucl. Med., 2000, vol. 27, pp Rosen, M.A., Jones, R.M., Yano, Y., and Budinger, T.F., J. Nucl. Med., 1985, vol. 26, pp Mishani, E., Ben-David, I., and Rozen, Y., Nucl. Med. Biol., 2002, vol. 29, pp Vasil ev, D.A., Kiselev, M.Yu., Korsakov, M.V., and Horti, A.G., Radiokhimiya, 1992, vol. 34, no. 3, pp Zlobin, V.A., Bukreeva, V.A., Kuznetsov, P.E., et al., Khim.-Farm. Zh., 2000, vol. 34, no. 5, pp

Product Description and Specification

Product Description and Specification Catalog No 7 Synthra [ 11 C]Choline Description Synthra [ 11 C]Choline is a completely automated synthesis system for routine production of [ 11 C]choline. Using in-target